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The Moto X got "top honors, by a country mile" in the research firm's report, which noted the handset garnered more than 35 percent higher average throughput than the other devices across all correlated and uncorrelated test scenarios. In addition, the Moto X could sustain a data call at signal levels as much as 9 dB lower than levels for the other devices tested.

Those other devices include the Samsung Galaxy S4, Samsung Galaxy Note II, HTC One and LG G2, which were tested over the 700 MHz Band 17 used by AT&T (NYSE:T). The Moto X was tested on 700 MHz Band 13, used by Verizon Wireless (NYSE:VZ).

Michael Thelander, Signals Research's founder and CEO, said use of different bands did not cause differences in the devices' performance, given that Band 13 and Band 17 are fairly close together on the spectrum chart. "I'm confident that if we had tested a Moto X phone that supported Band 17 in Band 17 that the results would be largely the same," told FierceWirelessTech.

Signals Research shows throughput versus orientation angle by device (-90 dBm) in this chart. Smartphone D (designated by the purple line) is the Moto X. Signals Research declined to specify which smartphones were represented by the other lines.The figure shows 12 of the 860 results for each phone. The power level selected for this figure was in the mid-range of the values used.

At a Qualcomm (NASDAQ:QCOM) modem event in November, Thelander said that when it comes to accepting devices for use on their networks, mobile carriers are more focused on compliance with specifications than performance. His firm's new over-the-air benchmark study reinforces the notion that regardless of how robust of a network an operator builds, the customer experience will be dramatically impacted by the quality of devices employed on it.

And crummy device connectivity impacts more than just the individual user carrying the poorly performing device. "If you've got a device out there that's making inefficient use of resources, it's impacting everybody else on the network," Thelander said.

All of the tested devices, which each used a Qualcomm chipset, were commercially procured and none of the handset vendors or operators had prior knowledge of the test campaign. Signals Research also collected preliminary data from Apple's (NASDAQ:AAPL) iPhone 5s but did not report results due to higher layer issues that were encountered, Thelander said.

Regarding the five devices for which results were reported, each was tested for about 14.4 hours in an anechoic chamber using a variety of angles and some 35 power levels. There were about 860 tests for each phone with each test lasting 60 seconds. The tests were conducted with equipment and other support from Spirent Communications and ETS-Lindgren.

The disparity in the devices' performance was partly a reflection of how well the devices leveraged MIMO capabilities. "Even when conditions warrant the use of MIMO, its effectiveness is highly dependent on the mobile device," Thelander said.

Signals Research separately tested two of the smartphones using only Transmission Mode 2 (TM2)--that is, base station transmit diversity mode--rather than the TM3 open-loop MIMO mode.

Results proved the incremental performance benefits of MIMO under ideal conditions. But they also showed that MIMO can result in a slightly negative performance when faced with poorer channel conditions or lower signal strength, implying that the network and device "probably should have reverted to transmit diversity instead," Thelander said. "Sometimes MIMO gets used more than it should," he added.

"I'm a huge fan of MIMO and I know that it works, but it isn't a panacea," Thelander said.

Signals Research also tested the impact of a plastic protective cover on a higher-end smartphone. Initial results indicate that there could be a modest degradation, perhaps 10-15 percent, in performance at higher signal levels when MIMO is being used.

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